Phosphoric acid is generally produced by the digestion of phosphate rock with concentrated sulfuric acid in a slurry of recycled phosphoric acid. This reaction also generates solid gypsum (CaSO4) particles, which are typically removed by filtration. In subsequent steps the phosphoric acid is clarified and concentrated by thermal evaporation to drive off more than half the water from the crude filtrate.
Flocculants are added to the crude phosphoric acid gypsum slurry to increase particle size, which improves settling and filtration rate. Traditional flocculants used for this application are supplied as a liquid concentrate or dry powder. The dissolution kinetics of traditional powders and the emulsion inversion kinetics of liquid concentrates prevent their direct feed to the crude slurry. A crude phosphoric acid gypsum slurry reaches the filtering stage in tens of seconds to a few minutes after the flocculant feed point, while emulsions and powders require tens of minutes to a few hours for dissolution or inversion. Thus, a concentrate or powder flocculant is diluted with water at the site to yield a working solution (typically containing 0.5% to 1% of the concentrate) that is subsequently fed into the process stream. Traditional flocculant make-down systems consist of a mixing tank to which concentrate or powder is added with water. After sufficient mixing the dilute flocculant is transferred to a storage tank from which the dilute solution is fed to the process. Use of traditional dilute flocculant solution accounts for about 0.5% to 1% of the total volume of water present in the crude phosphoric acid slurry. A typical phosphoric acid production facility will use 5-10 million gallons of water per year for flocculant make-down. After addition, this water must be removed by thermal evaporation in subsequent processing stages requiring use of substantial thermal energy.
Polymer conformation is highly influenced by solution composition. Turro and Arora (J. Phys. Chem. B, Vol. 109 No. 44, 2005, 20714-20718) note that an acrylic acid polymer is tightly coiled at low pH, but raising the pH ionizes the carboxylic acid groups and leads to stretching or swelling of the polymer. The sensitivity of polymer conformation to the presence of dissolved ions is well known, as is the tendency of polymers to bind to the surface of suspended particles. Thus, to ensure that polymers are in a predictable and optimal conformation, traditional polymeric flocculants are made down with raw water rather than low pH process water containing relatively high levels of dissolved solids and suspended particles.
U.S. Pat. No. 4,263,148, to Symens et al., discloses a process for removing colloidal and non-colloidal humic matter from an aqueous solution of phosphoric acid. The process includes the step of treating the phosphoric acid with a flocculant. The disclosure requires that the flocculant first be made down with water to a concentration of 0.1% to 0.5% prior to its further dilution with phosphoric acid to about 0.05% and subsequent addition to the phosphoric acid production process.
In contrast to traditional flocculants, quick inverting flocculant emulsions (e.g., Nalco Pol-EZ) can be added directly to the crude phosphoric acid slurry without the need for dilution. These products offer the advantage of eliminating traditional flocculant dilution water as well as the complicated expensive feed systems that are also associated with traditional flocculants. In some cases, for example, when the required feed point is too near the slurry discharge to the filter, the time is insufficient to allow these emulsions to undergo rapid inversion and adoption of an optimal polymer conformation within the process. As a result, the emulsion may not completely invert or the polymer may be trapped in a poor performing conformation when it binds to the gypsum surface. This results in less than optimal flocculant performance.
Accordingly, there is a need for optimizing the addition of flocculant to the phosphoric acid production process. Desirably, the addition of flocculant will minimize the introduction of water into the phosphoric acid production process. More desirably, the flocculant will be added to the phosphoric acid production process by mixing the flocculant with clarified phosphoric acid that is recycled from the phosphoric acid production process.